FIG. 1 is a block diagram illustrating an information storage system (disk drive) 110 according to of the prior art. Disk drives have one or more disks 111 on which ferromagnetic thin materials are deposited. The disk drive includes data recording disk 111, pivoting actuator arm 113, and slider 112 that includes a read head and a write head. The functional blocks include servo system 90, read/write electronics 114, interface electronics 115, controller electronics 116, microprocessor 117, and RAM 118. A disk drive can include multiple disks stacked on a hub that is rotated by a disk motor, with a separate slider for each surface of each disk.
The term servo wedge 120 will be used to mean the contiguous set of servo fields extending from ID to OD on the disk. Disk 111 will typically have multiple servo wedges 120 arranged radially around the disk, but only two are shown for simplicity. Information recorded on the disks is generally organized in concentric tracks. As part of the manufacturing process permanent servo information is recorded on the disks that provides information to the system about the position of the heads when the disks are rotating during operation. The servo data on the disk provides several fundamental functions and is conventionally arranged in distinct fields in each of the plurality of servo wedges angularly spaced around the disk.
FIG. 2B illustrates the fields in a selected servo ID (SID) 21. The preamble precedes Servo Address Mark (SAM) which is a timing mark which is used to synchronize data within the servo fields, and also provides timing information for write and read operations in the data portions of the disk. Second, the SID supplies a multi-bit digital field, which provides a coarse track-ID (TID) number and additional information to identify the physical SID number. The TID is typically written in Gray code as the presence or absence of recorded dibits. During seek operations, when the head is moving across tracks, the head can typically only read a portion of the Gray-code in each TID. The Gray-code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek.
The SID also supplies a position error field, which provides the fractional-track Position Error Signal (PES). Auxiliary functions, such as amplitude measurement or repeatable run-out (RRO) fields are sometimes also used. During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track. The typical PES patterns include either two or four bursts that are identical sets of high frequency magnetic flux transitions. FIG. 2B shows an example using only two PES bursts. The PES bursts are arranged in a pattern which generates a signal in the read head that is a function of the position of the read in relation to the centerline of the track. For example, the A and B bursts can be radially offset from each other by a half a track width and are sequential in the circumferential direction. Unlike the track-ID (TID) field number, the conventional PES bursts do not encode numerical information. The PES burst pattern is repeated for each set of two or four tracks, so only local information is provided. Variations of the standard PES burst pattern have been described such as the dual frequency, dual burst servo patterns described by Serrano, et al. in U.S. Pat. No. 6,078,445.
The write-to-read gap 23 is included to allow for the physical separation between the write head 32 and the read head 33 in slider 31 and to provide the time/distance needed to switch from writing data to reading the next servo sector ID (SID) 21. (See FIG. 2A). The bulk of the write to read gap is caused by the physical separation between the writer and reader. In most head designs, the reader leads the writer, so when the writer reaches the end of the data sector, the reader is already some distance beyond the end of the data sector. In addition some gap is needed to allow for the time needed for the drive's control systems to switch from writing to reading, but this switching gap is much smaller than the physical writer to reader separation. Accordingly servo systems have typically included a gap 23 in the track format between the end of a writable data sector and the start of the following servo sector information. A complicating factor in minimizing the needed gap is that the geometrical relationship (skew) between the heads and the track varies with the position of the mechanical actuator that move the slider with the heads in an arc across the disk surface.
U.S. Pat. No. 7,551,379 by Yu, et al. (Jun. 23, 2009) describes a system in which the write element leads the read element in the tangential direction of rotation of the magnetic disk. The servo sector information is arranged such that information that is not needed for write operation is placed at the end of the servo sector. In this way, the servo read operation can be terminated sooner and the write operation can initiate sooner after going over the servo sector. The write element in a write operation writes data to the data sector of a track until an end of the data sector before reaching a front end of a servo sector following the end of the data sector. The read element reads information in the servo sector needed for the write operation. The write element starts writing data in a next data sector following the servo sector after the write element reaches the next data sector and after the read element has read all information in the servo sector needed for the write operation.
In U.S. Pat. No. 6,104,568 to Drouin, et al. (Aug. 15, 2000) a pattern of odd even servo sectors is described in which the odd SIDs have an abbreviated TID Gray code.
PCT application WO/1999/006992 by Zaharris describes a pattern of servo fields that include mini-servo fields with shorter PES bursts that are placed in between normal servo sector SIDs around a track.
Each of these servo related functions including the write-to-read gap typically consumes a relatively independent portion of each track in prior art servo systems. Typically, these servo related fields can consume a significant portion of the recording surface of the disk and are an attractive target for reduction.